1. Technical Field
This document relates to an organic light emitting diode display device capable of compensating the threshold voltage of a driving TFT.
2. Description of the Related Art
The demand for various types of display devices for displaying an image is increasing. Various flat panel displays, such as a liquid crystal display, a plasma display panel, and an organic light emitting diode (OLED) display, have been recently used. Out of the various types of flat panel displays, the OLED display has excellent characteristics including a low voltage drive, a thin profile, a wide viewing angle, and a fast response time. Especially, an active matrix type OLED display for displaying an image on a plurality of pixels, which are arranged in a matrix form, has been widely used.
A display panel of the active matrix type OLED display comprises a plurality of pixels arranged in a matrix form. Each of the pixels comprises a scan thin film transistor (TFT) for supplying a data voltage of a data line in response to a scan signal of a scan line and a driving TFT for adjusting the amount of current supplied to an organic light emitting diode in accordance with a data voltage supplied to a gate electrode. The drain-source current Ids of the driving TFT supplied to the organic light emitting diode can be represented by Equation 1:Ids=k′·(Vgs−Vth)2  [Equation 1]
where k′ represents a proportionality coefficient determined by the structure and physical properties of the driving TFT, Vgs represents the gate-source voltage of the driving TFT, and Vth represents the threshold voltage of the driving TFT.
The threshold voltage Vth of the driving TFT of each of the pixels may have a different value due to a shift in the threshold voltage Vth caused by degradation of the driving TFT. In this case, the drain-source current Ids of the driving TFT depends upon the threshold voltage Vth of the driving TFT. Hence, the current Ids supplied to the organic light emitting diode differs from pixel to pixel even if the same data voltage is supplied to each of the pixels. Accordingly, there arises the problem that the luminance of light emitted from the organic light emitting diode of each of the pixels differs even if the same data voltage is supplied to each of the pixels. To solve this problem, various types of pixel structures for compensating the threshold voltage Vth of the driving TFT have been proposed.
FIG. 1 is a circuit diagram showing a part of a diode-coupled threshold voltage compensation pixel structure. FIG. 1 depicts a driving TFT DT supplying current to an organic light emitting diode and a sensing TFT ST coupled between a gate node Ng and drain node Nd of the driving TFT DT. The sensing TFT ST allows for a connection between the gate node Ng and drain node Nd of the driving TFT DT during a threshold voltage sensing period of the driving TFT DT so that the driving TFT DT is driven by a diode. In FIG. 1, the driving TFT DT and the sensing TFT ST are illustrated as N-type MOSFET (Metal Oxide Semiconductor Field Effect Transistors).
Referring to FIG. 1, the gate node Ng and the drain node Nd are coupled during the threshold voltage sensing period in which the sensing TFT ST is turned on, thereby allowing the gate node Ng and the drain node Nd to float at substantially the same potential. If a voltage difference Vgs between the gate node Ng and a source node Ns is greater than a threshold voltage, the driving TFT DT forms a current path until the voltage difference Vgs between the gate node Ng and the source node Ns reaches the threshold voltage Vth of the driving TFT DT, and as a result, the voltage of the gate node Ng and the drain node Nd is discharged. However, if the threshold voltage Vth of the driving TFT DT is shifted to a negative voltage, the voltage difference Vgs between the gate node Ng and the source node Ns cannot reach the threshold voltage Vth of the driving TFT DT, even if the gate node Ng goes down to 0 V, because the threshold voltage Vth of the driving TFT DT is lower than 0 V. Consequently, if the threshold voltage Vth of the driving TFT DT is shifted to a negative voltage, the threshold voltage Vth of the driving TFT DT cannot be sensed. A negative shift refers to shifting the threshold voltage Vth of the driving TFT DT to a voltage lower than 0 V when the driving TFT DT is implemented as an N-type MOSFET. The negative shift often occurs when a semiconductor layer of the driving TFT DT is formed of an oxide.